I was playing around with the HList library from the paper... Strongly typed heterogeneous collections http://homepages.cwi.nl/~ralf/HList/ ...and I thought I'd try to fold the composition function (.) through a heterogeneous list of functions, using hFoldr...
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
import CommonMain
main = print $ comp "abc"
test = HCons ((+1)::(Int->Int)) (HCons ((*2)::(Int->Int)) (HCons length HNil))
comp = hFoldr (.) id test
instance Apply (a -> b -> c -> d) (a, b) (c -> d) where apply f (a,b) = f a b
...but it fails with the following type error... ]Compiling Main ( compose.hs, interpreted ) ] ]compose.hs:10:7: ] No instances for (Apply ((b -> c) -> (a -> b) -> a -> c) ] (Int -> Int, r) ] ([Char] -> a3), ] Apply ((b -> c) -> (a -> b) -> a -> c) (Int -> Int, r1) r, ] Apply ((b -> c) -> (a -> b) -> a -> c) ([a2] -> Int, a1 ->a1) r1) ] arising from use of `hFoldr' at compose.hs:10:7-12 ] Probable fix: ] add an instance declaration for (Apply ((b -> c) -> (a -> b) -> a -> c) ] (Int -> Int, r) ] ([Char] -> a3), ] Apply ((b -> c) -> (a -> b) -> a -> c) ](Int -> Int, r1) r, ] Apply ((b -> c) -> (a -> b) -> a -> c) ]([a2] -> Int, a1 -> a1) r1) ] In the definition of `comp': comp = hFoldr (.) id test ...Anyway, I couldn't quite tell whether I was using hFoldr incorrectly, or if I needed to have more constraints placed on the construction of "test", or if needed some sort of type-level function that resolves... Apply ((b -> c) -> (a -> b) -> a -> c) ...into (a -> c), or something else altogether. I figured someone might be able to help point me in the right direction. Thanks, Greg Buchholz